Gas tube having an emissive shield



April 5, 1966 B. o. BAKER ETAL 3,244,925

GAS TUBE HAVING AN EMISSIVE SHIELD Filed April l0, 1963 2 Sheets-Sheet 1 F lg. 1

INVENToRS 75a F' 4,24 KENT (/05574 llx p H T n NEYS April 5, 1966 B. o. BAKER ETAL GAS TUBE HAVING AN EMISSIVE SHIELD 2 Sheets-Sheet 2 Filed April 10, 1963 wverrmvzs 'B OPFo 39x 2 ROEERT Josflfi WHGLPM/ 38 4a fight". 0

FITTDRNEYS United States Patent Claims priority, application Great Britain, Apr. 30, 1962,

27/62 Claims. (Cl. 313-411 This invention relates to gas filled thermionic valves.

According to the invention, a gas filled thermionic valve includes an anode, a thermionic cathode which has a generally cylindrical electron-emissive surface, and a metal structure which is generally in the form of at least part of a hollow cylinder having a number of inwardly projecting portions extending along at least part of the length of the cylinder, said metal structure being disposed at least partly around the thermionic cathode with its axis substantially parallel to that of said electron-emissive surface and the arrangement being such that, during the manufacture of the valve, electron-emissive material derived from the thermionic cathode is deposited on the surface of said portions over an area at least twice as great as the area of said electron-emissive surface, and that, in operation of the valve, emission from those regions of said portions on which electron-emissive material has been deposited may contribute appreciably to :the total anode current of the valve.

According to one aspect of the invention, in a thermionic valve in accordance with the immediately preceding paragraph, said material is deposited on the surface of said portions over an area at least four times as great, and preferably at least ten times as great, as the area of said electron-emissive surface.

formed by a body of porous refractory metal within the pores of which is dispersed at least one alkaline earth metal compound.

It has been found that, with a gas filled thermionic valve in accordance with the present invention, it is pos sible to arrange that, in operation, the greater part of the peak anode current of the valve is contributed by emission from the deposited material; thus, with such an arrangement, the heater power required to enable a given anode current to be passed by the valve is considerably less than would have been required if said metal structure had not been provided with said inwardly projecting portions.

One arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a part sectional elevation, shown partly broken away, of a thyratron adapted to operate with a peak anode current of about 2,500 amperes; and

FIGURE 2 is a sectional plan view of the thyratron, the section being taken along the line lI--II of FIG- URE 1.

Referring to the drawings, the thyratron has a metal Patented Apr. 5, 1966 envelope 1 filled with deuterium at a pressure of 0.3 millimetre of mercury, the main body of the envelope 1 being in the form of a circular cylinder having an inner diameter of about 6 centimetres and a length of about 10 centimetres. The electrode system of the thyratron is housed in the main body of the envelope 1 and includes an anode 2, a control electrode 3 and a thermionic cathode 4.

The anode 2 is in the form of a metal disc, 5 centimetres in diameter, which is disposed adjacent one end of the main body of the envelope 1 with its main faces perpendicular to the axis of the envelope 1. An electrical contact for the anode 2 is provided in the form of a metal rod 5 one end of which is secured to a central region of the anode 2, the rod 5 extending through a closure member 6 which serves to close the relevant end of the envelope 1. The closure member 6 includes an electrically insulating support 7 which serves to insulate electrically the anode 2 from the envelope 1.

The control electrode 3 includes three metal discs 8, 9 and 10 which are arranged with their main faces perpendicular to the axis of the envelope 1, the peripheries of the discs 8, 9 and 10 being secured to the wall of the main body of the envelope 1, and the main faces of the centre disc 9 being respectively in contact with the adjacent main faces of the other discs 8 and 10. A pair of curved slots 11 are symmetrically formed in each of the discs 8, 9 and 10, the slots 11 in the centre disc 9 being wider than the other slots 11, and the slots 11 in the disc 8 nearest the anode 2 being disposed nearer the centre of the envelope 1 than are the slots 11 formed in the disc 10; the purpose 'of the slots 11 is to enable a discharge struck between the cathode 4 and anode 2 in operation of the valve to pass through the control electrode 3. A disc-like batlle 12, 3.7 centimetres in diameter, is disposed on that side of the control electrode 3 remote from the anode 2 and issecured to the disc 10 by means of an electrically insulating support 13, the main faces of the batlle 12 being arranged perpendicular to the axis of the envelope 1.

The cathode 4 consists essentially of a circular cylinder 14 made of porous tungsten impregnated with alkaline earth metal compounds, the upper end (with reference to FIGURE 1) of the cylinder 14 being closed, and the cylinder 14 having an outer diameter of 1.2 centimetres and a length of 1.3 centimetres. The cylinder 14 is disposed coaxially inside the main body of the envelope 1 with its upper end facing, and spaced about 2.5 centimetres from, the baffle 12. A heating element 15 is housed inside the cylinder 14, one end of the heating element 15 being secured to the upper end of the cylinder 14. The other end of the heating element 15 is sealed through a metal eyelet 16 which is in turn sealed through an electrically insulating disc 17; the disc 17 is secured to the lower end of the cylinder 14 by means of an apertured metal cap 18. The total electron-emissive surface area of the cathode 4 is about 6 square centimetres, and

of this the greater part is formed by the curved emissive surface of the cylinder 14 which has an area of about 5 square centimetres.

The cathode 4 is coaxially surrounded by a circular V molybdenum cylinder 19 which has an inner diameter of tion in the envelope 1 by virtue of that end of the cylinder 19 remote from the anode 2 being secured to a ceramic disc 20 secured across the lower end of the main body of the envelope 1. The upper end of the cylinder 19 is formed integral with the wider end of a frusto-conical molybdenum member 21 the narrower end of which has a diameter of 3 centimetres and lies in a plane spaced about 1.5 centimetres from the upper end of the cathode 4. The cylinder 19 acts as a heat shield for the cathode 4, while the conical member 21, in combination with the battle 12, serves to reduce the evaporation of electron-emissive material from the cathode 4 on to the control electrode 3 during the manufacture of the valve and while the valve is in operation. The cathode 4 is held in positionrinside the cylinder 19 by means of two metal spiders 22 each consisting of three arms 23 connected together by means of an apertured central portion 24, the free ends of the arms 23 of each spider 22 being secured to the cylinder 19; the central portion 24 of one of the spiders 22 is secured to the upper end of the cathode 4 by means of a molybdenum nut 25 and a screw 26 while the central portion 24 of the other spider 22 is secured to an outwardly projecting circumferential flange 27 formed integral with the lower end of the cathode 4. It will be appreciated that the spiders 22 also serve to connect electrically the cylinder 19 to the cathode 4.

A series of 24 molybdenum fins 28, each 0.1 millimetre thick, are secured to, and project radially inwardsfrom the inner surface of, thecylinder 19, the fins 28 being spaced at equal intervals around the axis of the cylinder 19; for the sake of clarity, only two fins 28 are shown in FIGURE 1. Each fin 28 extends for a length of 3 centimetres from the upper end of the cylinder 19 and has a width of 1 centimetre; thus each fin 28 has a surface area of about 6 square centimetres so that the total surface area of the fins 28 is about 144 square centimetres.

A metal disc 29 is secured across the interior of the cylinder 19 level with the lower ends of the fins 28, the disc 29 serving to support a barretter 30 housed in the lower part of the cylinder 19. A number of holes 31 are formed in the wall of the lower part of the cylinder 19 for the purpose of reducing the conduction of heatin operation from the fins 28 to the envelope 1 via the cylinder 19 and the disc 20.

The lower end of the main body of the envelope 1 is partially closed by the ceramic disc 20 which separates the main body of the envelope 1 from an'end compartment 32 in which is housed a replenisher (not shown) for the gas filling of the valve, the main body communicating with the end compartment 32 via a number of holes such as the hole 33 formed in the disc 20. Electrical leads 34 for the cathode 4, the bafile 12, the heating element 15, the barretter 30, and a heating element incorporated in the replenisher are sealed through the wall of the end compartment 32.

'ried on for two or three hours, and during this process electron-emissive material including barium is evaporated from the cathode 4 and deposited on substantially the whole of the surface of the fins 28 and also on the inner surface of the cylinder 19. Finally, the envelope 1 is filled with deuterium and then sealed.-

It has been found that, in operation of the valve described above using a heater power of about 100 watts, the fins 28 contribute appreciably to the total anode current of the valve provided that the valve is arranged to pass short current pulses (which typically may have a duration of microseconds); in fact, it has been found that for pulses of this duration the fins 28 contribute the greater part of the peak anode current. The inner surface of the cylinder 19 also contributes to some extent to the anode current provided that the valve is arranged to pass short current pulses, but since the area of this surface is considerably less than the surface area of the fins 28 the current contributed by the inner surface of the cylinder 19 is considerably less than that contributed by the fins 28. It is thought that the temperature of the fins 28 does not reach a sufficiently high value for the electron emission from the fins 28 to be due to thermionic emission, and it is thought that such emission may be photo-electric emission or may be caused by positive ion bombardment.

It has been found that if the valve described above is arranged to pass long current pulses (having a duration of greater than about 1 millisecond), then it may be necessary to increase the heater power slightly so as to increase the temperature of the fins 28 if the fins 28 are to contribute appreciably to the peak anode current.

It has been found that with the valve described above the heater power required to enable the valve to pass current pulses having a magnitude of 2500 amperes and a duration of 5 microseconds is only about watts. This may be compared with a gas filled thermionic valve which also has a heat shield surrounding the cathode and which is adapted to pass current pulses of similar amplitude and duration, but in which the heat shield is not provided with any inwardly projecting portions; in this case, it was found that the heater power required to enable the valve to pass such current pulses was about 300 watts. Thus, it will be appreciated that the present invention provides a gas filled thermionic valve in which a considerable saving in heater power may be achieved without any decrease in the current rating of the valve.

It should be understood that, in an alternative arrangement to that described above, the cylinder 19 need not be electrically connected to the cathode 4 internally of the valve. Instead, a separate electrical lead could be provided for the cylinder 19, so that in operation the cylinder 19 and cathode 4 could be electrically connected together externally of the valve.

We claim:

1. A gas filled thermionic valve, including: an anode; a thermionic cathode which has a generally cylindrical electron emissive surface; a metal structure which is generally in the form of at least part of a hollow cylinder having a number of inwardly projecting portions extending along at least part of the length of the cylinder; and a coating of electron emissive material on the surface of said portions, the coating having an area at least twice as great as the area of said electron emissive surface, said metal structure being disposed at least partly around the thermionic cathode with its axis substantially parallel to that of said electron emissive surface and with the surface of said portions adjacent said cathode whereby the electron emission from said surface contributes appreciably to the total anode current of the valve.

2. A thermionic valve according to claim 1, having a gas filling constituting hydrogen.

3. A thermionic valve according to claim 1, in which said metal structure and the cylindrical electron-emissive surface of the cathode are substantially coaxial.

'loaded with a material incorporating at least one alkaline earth metal compound.

7. A thermionic valve according to claim 1, in which said portions are in the form of fins.

8. A thermionic valve according to claim 7, in which the fins extend generally parallel to the length of said metal structure.

9. A thermionic valve according to claim 1 wherein said coating has an area at least four times as great as the area of said electron emissive surface.

10. A thermionic valve according to claim 9 wherein said coating has an area at least ten times as great as the area of said electron emissive surface.

6 References Cited by the Examiner UNITED STATES PATENTS 2,572,881 10/1951 Rothstein 313-2l2 X 2,953,705 9/1960 Cook 3l32l3 X GEORGE N. WESTBY, Primary Examiner.

D. E. SRAGOW, Assistant Examiner. 

1. A GAS FILLED THERMIONIC VALVE, INCLUDING: AN ANODE; A THERMIONIC CATHODE WHICH HAS A GENERALLY CYLINDRICAL ELECTRON EMISSIVE SURFAC; A METAL STRUCTURE WHICH IS GENERALLY IN THE FORM OF AT LEAST PART OF A HOLLOW CYLINDER HAVING A NUMBER OF INWARDLY PROJECTING PORTIONS EXTENDING ALONG AT LEAST PART OF THE LENGTH OF THE CYLINDER; AND A COATING OF ELECTRON EMISSIVE MATERIAL ON THE SURFACE OF SAID PORTIONS, THE COATING HAVING AN AREA AT LEAST TWICE AS GREAT AS THE AREA OF SAID ELECTRON EMISSIVE SURFACE, SAID METAL STRUCTURE BEING DISPOSED AT LEAST PARTLY AROUND THE THERMIONIC CATHODE WITH ITS AXIS SUBSTANTIALLY PARALLEL TO THAT OF SAID ELECTRON EMISSIVE SURFACE AND WITH THE SURFACE OF SAID PORTIONS ADJACENT SAID CATHODE WHEREBY THE ELECTRON EMISSION FROM SAID SURFACE CONTRIBUTES APPRECIABLY TO THE TOTAL ANODE CURRENT OF THE VALVE. 